Season Five, also known as Day 5, of the television series 24 premiered on January 15, 2006 and aired its season finale on May 22, 2006. The Season Five storyline starts and ends at 7:00 a.m. - the same time frame as the previous season. It is widely considered to be the series' most acclaimed season by fans and critics alike, scoring the show's highest ratings and receiving twelve Primetime Emmy Award nominations with five wins, including Outstanding Drama Series.

Season Five is set 18 months after Season Four. Former Counter Terrorist Unit agent Jack Bauer begins the season, working as a day-to-day laborer at an oil refinery under the alias "Frank Flynn" in Mojave, California. On a monumental day in Charles Logan's presidency, an anti-terrorism treaty with Russian President Yuri Suvarov is about to be signed when events take a tragic turn.

Season 5 is one of the 24 seasons that can be divided into three main acts:

In the first act, two murders force Jack to reveal himself. He attempts to save innocent people from a plot involving Russian separatists that is rapidly unfolding.

In the second act, the terrorists gain control of nerve gas canisters with help from corrupt officials.

In the final act, Jack discovers how deep the conspiracy goes and fights against numerous threats to get hold of a recording that implicates President Logan.

The terrorists are Russian separatists in contact with an official named James Nathanson. They take 40 people hostage in attempt to dissuade President Logan from signing a landmark anti-terrorism treaty with Russian President Yuri Suvarov. Terrorists, led by Vladimir Bierko, use the plot as a diversion to steal 20 canisters of VX nerve gas. While searching for the nerve gas, Jack discovers the involvement of James Nathanson and White House Chief of Staff Walt Cummings. They plan to release nerve gas on Russian soil and to use that as an excuse to invoke the military clauses of the treaty, allowing Logan to secure American petroleum interests in Central Asia. David Palmer was killed because he was beginning to find out about their plan.

The nerve gas is deployed at Sunrise Hills Shopping Mall (resulting in 10-20 fatalities out of eight-nine hundred shoppers) and Tyler Memorial Hospital (unsuccessfully due to the actions of CTU and Curtis Manning). Jack questions a former co-worker of his, Christopher Henderson, whom he believes is willing to help. Instead Henderson locks Jack in a room with a bomb nearly killing him. Jack has Henderson brought to CTU and tortured where he refuses to reveal additional knowledge about the conspiracy. When terrorists break into CTU and release nerve gas there, Christopher Henderson, Tony Almeida, Jack Bauer, Kim Bauer, Chloe O'Brian, Audrey Raines and others are spared in sealed safe-rooms. 40% of the personnel including Edgar Stiles and Lynn McGill are killed. When CTU is decontaminated, Jack rushes to the medical room and sees that Henderson killed Tony and escaped.

The fallout from this attack convinces the Vice President to absorb CTU into Homeland Security (DHS). CTU and Homeland Security agents race to find the location of the remaining nerve gas canisters. This turns out to be Wilshire Gas Co. By incinerating the pipelines, Jack stops the nerve gas from being released and captures Vladimir Bierko. Through the assistance of Wayne Palmer (David Palmer's brother) and Evelyn Martin (Special Assistant to the First Lady), Jack discovers that President Logan is responsible for the attacks. He is at the center of a conspiracy involving Cummings, Henderson, Nathanson, a mysterious man known as Graem and a group of men who monitor and influence the actions of Logan from an undisclosed location.

Jack and Wayne retrieve a recording that implicates Logan from a bank vault and attempt to turn it over to U.S. Secretary of Defense James Heller. Things do not go as planned and Christopher Henderson uses Audrey as leverage to get the recording from Jack. Jack and Audrey recapture Henderson but they find that he has passed the recording on to a co-conspirator aboard a diplomatic flight. Jack boards the plane and reclaims the recording by taking the co-pilot hostage. This triggers a response from Charles Logan who tries to have the plane shot down, citing that it could be used against civilians. This causes others in Logan's inner circle to become suspicious of his motives. Jack Bauer lands the plane and delivers the recording to CTU only to have it destroyed by a traitor from DHS.

Bierko escapes CTU custody, and uses his last nerve gas container to take over a Russian submarine and gain control of twelve multiple-warhead non-nuclear missiles, each one capable of wiping out approximately seven city blocks. After cutting a deal with Christopher Henderson, Jack and Henderson together stop the attack from the submarine, and kill Bierko and his men. During a final standoff, Jack kills Henderson to get revenge for his friends. With the help of the First Lady, Mike Novick, Aaron Pierce, and Chloe, Jack is able to place a listening device on Logan which informs the U.S. Attorney General of his deception. At a memorial service for David Palmer, Charles Logan is taken away by U.S. Marshals. However, Jack Bauer is captured as well by Chinese agents. The season ends with a bound and tortured Jack on a ship headed for China.

A 10-minute prequel to the season was available with the fourth season DVD.[3] The prequel, which takes place one year after Day 4, shows Jack and Chloe meeting in Chicago, Illinois. Based on computer intrusion that happened when Chloe accessed Jack's autopsy report, she expresses concern that others might know Jack is still alive. This is confirmed when he tries to leave the meeting place and gets chased by agents in a black car. It was broadcast on Sky One in the United Kingdom before the showing of the fifth season.

The trailer for the fifth season recapped the Season 4 ending and emphasized that the show would be continuously aired. Many scenes shown were from the Season 5 premiere but a scene that shows an aerial view of Jack Bauer was unique to the trailer. Such a scene would not have been used in the show because the camera department prefers to shoot all scenes at eye level.[4]

This season is notable for the high number of major characters killed off. Significant deaths included David Palmer, Michelle Dessler, and Edgar Stiles. Actor Dennis Haysbert was against the killing of David Palmer and refused to film the scene. He relented after a meeting with the showrunner, though he still disapproved of the storyline.[5] Spoilers released by Ain't It Cool News revealed that two major characters including a politician would die in the premiere episode.[6]Tony Almeida was seeming killed, but was revealed to be alive in the seventh season. Show runner Howard Gordon mentioned in an interview that they purposely filmed Tony's death in a way that would allow his eventual return.[7] Guest characters who died were Walt Cummings and Lynn McGill. The life of Aaron Pierce was threatened, but Martha Logan helped save him.

"7:00 a.m. – 8:00 a.m." was the most watched episode and premiere in 24 '​s history.[8] Reviews of "7:00 a.m. – 8:00 a.m." were almost universally positive, with a score of 89/100 from Metacritic.[9] In positive light The Hollywood Reporter called the episode, "heart-pounding, mesmerizing adventure unlike anything else up or down the dial." while praising the new and returning actors, giving it a perfect 100,[10] likewise USA Today also gave the episode a 100 and praising Kiefer Sutherland, who portrayed Jack Bauer.[11]

The New York Times gave the fifth season a 70 out of a possible 100, saying, "all the villains, be they Islamic fundamentalists, drug smugglers, American oil brokers or Russian separatists, keep picking Los Angeles as a target for attack."[12]The Boston Globe gave it an 80, praising returning performances and strong plot.[13] Season five of 24 was nominated for twelve Primetime Emmy Awards, becoming the most nominated series.[14]

Eighteen months after faking his death, Jack Bauer (Kiefer Sutherland), who has assumed the name Frank Flynn and living with a mother and son family Diane and Derek Huxley, returns to Los Angeles after the four people who know he's alive come under attack. Former President David Palmer (Dennis Haysbert) is assassinated, as is Michelle Dessler (Reiko Aylseworth) while Tony Almeida (Carlos Bernard) is critically wounded. Chloe O'Brian is also targeted but manages to evade the assassins and contact Jack for help. President Charles Logan (Gregory Itzin) prepares to sign an important anti-terrorism treaty with the Russian President Yuri Suvarov (Nick Jameson). First Lady Martha Logan (Jean Smart) is disturbed about the morning's events. Jack, after rescuing Chloe, learns from one of the assassins that he's being framed for the attacks on Palmer, Tony and Michelle.

Jack and Chloe O'Brian (Mary Lynn Rajskub) infiltrate Wayne Palmer's (DB Woodside) penthouse at the hotel in hopes of finding a lead. At the penthouse Wayne Palmer walks in. After Jack manages to convince him of his innocence, Wayne helps Jack look for clues as to why David Palmer was targeted. Jack finds the information that David Palmer possessed that got him killed — a clue leading to Ontario Airport in Los Angeles. Martha tries to convince her husband that she had a conversation with Palmer the night before. As Jack heads to the airport to investigate, Walt Cummings (John Allen Nelson) is revealed to be a traitor and his ally — James Nathanson (Geraint Wyn Davies) — launches the next phase of his plan at the Ontario Airport.

Jack conceals himself inside the airport, surveying the Russian separatists and feeding information to Curtis Manning (Roger Cross) and his assault team. The terrorists continue to execute hostages in an attempt to make Logan give in to their demands to nullify the treaty. With help from Chloe back at CTU Jack detonates one of the terrorist's explosive vests to stall the executions. However the traitor Walt Cummings contacts Beresch, the leader of the terrorists and informs him of Jack's presence and his connection to one of the hostages, Derek Huxley (Brady Corbet). Beresch threatens to execute Derek, forcing Jack to give himself up. Martha discovers that all the traces of her conversation with Palmer might not have been erased and accesses the archives at the presidential retreat looking for a transcript.

Jack is captured by the terrorists, who use Derek Huxley as leverage to force him to compromise CTU's assault team by misinforming Curtis of the terrorist's position. Logan refuses to give in to the terrorists' demands and signs the treaty along with President Suvarov. Jack manages to communicate a duress signal but CTU initially do not recognize it as it is a defunct code from when Jack was active. Martha continues to try to convince her husband that the phone call from Palmer was real, but Cummings interferes, incapacitating her and stealing her copy of the phone transcript. A new representative from Division, Lynn McGill arrives at CTU and begins to undermine Bill Buchanan's (James Morrison) authority. McGill however, detects the duress signal and manages to warn Curtis, who is able to reposition his teams and outflank the terrorists. Beresch's men are killed and Beresch kills himself rather than be captured. Jack identifies a sleeper amongst the hostages and CTU begin their search for him. It turns out that a further objective of the attack on the airport was to provide the sleeper with an access card to a secure storage facility in one of the airport's hangars.

Jack returns to CTU and has an emotional reunion with Audrey Raines (Kim Raver), while Curtis Manning tracks the sleeper from the airport hostages and discovers the hangar which contained military-grade Sentox nerve gas. CTU begin their hunt for the Sentox. Cummings sends in an assassin to silence Jack, infiltrating CTU with help from Spenser Wolff, who is revealed to be working for Cummings. Chloe discovers this and has Spenser apprehended and interrogated. Jack is lured downstairs to medical and is attacked but survives the attempt on his life, killing the assassin in the process. Cummings, fearing exposure convinces Logan to send Martha away to be institutionalized when she tells them of the attacks against her.

Jack meets with Mike Novick (Jude Ciccolella) in hopes of gathering evidence against Cummings but Cummings has Secret Service intercept them, taking them both into custody. Martha begs Aaron Pierce (Glenn Morshower) for help while Cummings thinks that he is forcing Logan into aiding him with his complicity with the terrorists. Cummings reveals that he has a man inside with the terrorists who has set a tracker on the canisters of nerve gas. The intention is to remotely detonate the canisters once they have been transported back to the terrorist's camp, thus justifying US military presence in Central Asia and ensuring a steady flow of oil from the region for the benefit of the US. Buchanan butts heads with Lynn McGill (Sean Astin) over a decision to violate the President's orders, while the Russian separatists attempt to escape with the nerve gas. With the help of Pierce, Jack escapes and confronts Cummings who eventually reveals the location of the nerve gas. However the terrorists have discovered Cumming's ploy and have already escape with the nerve gas.

Jack learns of a man named Jacob Rossler, a programmer who has links to Ivan Erwich (Mark Sheppard). Rossler has agreed to reprogram the detonators on the canisters. Erwich threatens a mechanic at a garage to cut open the canisters to access the detonator. Logan, Mike and Martha discuss the best way to handle the Cummings situation, with Martha wanting to tell the public the truth. McGill has personal problems involving his sister, a drug-addict, that draw him away from work. McGill is attacked by his sister's boyfriend who steals his money and his CTU access card. Jack and Curtis infiltrate Rossler's apartment and apprehend him. At his apartment they also find a young Ukrainian girl Rossler has kept in captivity as a sex slave. A deal is authorized in exchange for Rossler's help, but the girl shoots Rossler, killing him.

Jack poses as Rossler, the programmer, and manages to trace one of the Sentox nerve gas canisters. Two of Erwich's henchmen take Jack to the target — a shopping mall and prepare to release it into the ventilation system. Buchanan and McGill advise Logan to allow this canister to be released to maintain Jack's cover, in order to track the terrorists back to the remaining 19 canisters which Logan authorizes. However Jack goes against the President's orders and attempts to intervene. Some of the gas is released, inflicting casualties but Jack is able to evacuate the mall and kill one of the terrorists. The other one manages to escape as CTU track him back to Erwich. However Erwich takes evasive actions and spots CTU moving in, before ordering his henchman to commit suicide to avoid capture. Logan and Martha argue when she learns that he intends to cover up the real reason for Cummings's suicide.

Vladimir Bierko (Julian Sands) kills Erwich, angry that he wasted a canister of gas. Jack is contacted by James Nathanson, Cummings' middleman and goes to meet him. However the terrorists having discovered their deception send a team to eliminate him. Nathanson is killed but Jack manages to fight off the terrorists and obtains a data chip from Nathanson before he dies. Chloe finds the name Omicron Corporation on the chip, to whose CEO Jack has ties. McGill, faltering under enormous pressure, goes on a warpath at CTU, putting personnel under surveillance and detaining Buchanan. Bierko demands the Suvarovs' motorcade route from Logan, threatening to release more of the Sentox nerve gas if he refuses. Martha demands Logan to stand up to the terrorists but he refuses. In an attempt to force the President to call back the motorcade and save the Suvarovs, Martha decides to go with them to the airport.

Jack meets Christopher Henderson (Peter Weller) at Omicron who has some history with Jack — Henderson was formerly head of CTU Field Ops and recruited Jack until Jack turned whistle-blower on Henderson after discovering evidence of corruption. Henderson initially protests his innocence and helps Jack, however this was all an act and Jack narrowly escapes death when Henderson attempts to cover his tracks with a bomb. Having detected evidence indicating a possible attack on the Suvarovs' motorcade, Audrey goes to McGill, who dismisses it as a unsubstantiated threat and refuses to devote anymore manpower to warn Secret service. Audrey, Chloe and Edgar go behind McGill's back to warn Secret Service but are discovered by McGill who is about to have them removed from their posts for insubordination until Audrey convinces Manning to have McGill removed instead. Curtis does this and reinstates Buchanan. Secret Service are warned just as Bierko's men attack the Suvarovs' motorcade. Thanks to the timely intervention of Aaron Pierce, the Suvarovs survive the attack.

Jack Bauer interrogates Henderson's wife at his home. He then tries to access Henderson's computer but Chloe is unable to hack into it. Henderson arrives home and is apprehended by Jack, who goes to extreme measures to get the information out of him, even shooting Henderson's wife in the leg. Bierko targets a hospital but CTU is able to get a warning out and evacuate the hospital. Curtis kills the terrorist and manages to seal the canister into an airtight containment apparatus. Meanwhile Tony Almeida (Carlos Bernard) wakes up and asks about Michelle's fate. Buchanan and the doctor's vague assurance that she is all right causes Tony to access the CTU server and discover that Michelle died in the explosion. Meanwhile, Martha tries to avoid her husband.

Jack has an emotional reunion with his daughter Kim Bauer (Elisha Cuthbert) at CTU. He then interrogates Henderson, but with no success. A Bierko associate reprograms and uses McGill's key card (which was stolen earlier in the day) to access CTU with a canister of nerve gas. McGill, upon being informed that his sister and her boyfriend have been murdered, confesses to Buchanan that they stole his key card. Buchanan initiates a lockdown, and although the terrorist is killed by Jack, they are too late to prevent the gas from being released. Several members of CTU are killed, including Edgar Stiles (Louis Lombardi) while Jack, Kim, Chloe, Buchanan and a small number of CTU personnel manage to seal themselves into some safe rooms. Meanwhile, Logan debates martial law with Vice President Hal Gardner (Ray Wise). Mike Novick objects, and enlists Martha's help to talk Logan out of it.

Jack and the rest of CTU take action to protect the remaining isolated areas while Chloe wrestles with her grief over Edgar's death. The fact that a corrosive agent had been added to the nerve gas means that it is slowly eating away at the seals of the isolated safe-rooms and forces them to take immediate action to slow it down so that Curtis can arrive with a Haz-Mat team. Lynn McGill sacrifices himself to shut down the controls of the ventilation system, allowing Chloe to override and slow down the pathogen, saving the lives of the survivors. Meanwhile Tony contemplates revenge on Henderson. Martha bolsters Logan's judgment as he faces political pressure from Gardner about imposing martial law. As Tony prepares to kill Henderson with a lethal injection, he hesitates and Henderson, waking up, plunges the needle into Tony, killing him.

Henderson manages to escape while CTU investigates a new lead on Bierko, but finds resistance from German intelligence officer Theo Stoller (Henry Ian Cusick) who is also tracking the same lead — Collette Stenger who has links to Bierko. Martha Logan is skeptical of the administration's move for a curfew. New Homeland Security officials led by Karen Hayes attempt to change operating protocol involving CTU after the Sentox attack. Meanwhile, Wayne has some critical information he has to get to Pierce but is attacked by armed men. Jack apprehends Stenger, having offered Stoller a copy of the NSA WET list much to the dismay of Karen Hayes. However after Stoller takes his leave the memory card self-destructs — Jack has deceived him and had no intention of handing over the WET list. During interrogation, Stenger implicates Audrey Raines.

Jack is forced to interrogate Audrey when he learns that she might be connected to Bierko and the terrorists. Although he is convinced of her innocence Karen Hayes is unconvinced and decides to push Audrey harder, forcing Jack and Chloe to search for evidence that Stenger is lying. Eventually Stenger admits that implicating Audrey was a ploy to throw CTU off, and informs them of Bierko's next target. Wayne and Pierce manage to link up and flee from armed men who have orders from Henderson to kill them. Bierko and his men execute the next phase of attacks by targeting a gas company and releasing the Sentox gas throughout Los Angeles. Jack and Curtis lead a TAC team to stop Bierko, but are too late to prevent the Sentox being released. Jack then sets C4 charges on the main gas pipe to destroy the gas before it can reach the main gas tank. Jack captures Bierko just as he is trying to escape.

Jack teams up with Wayne to help Palmer's source in the White House: Martha Logan's aide, Evelyn Martin (Sandrine Holt) whose daughter Amy has been kidnapped by Henderson in order to blackmail her to give up evidence. Gardner has a run-in with Wayne, and Jack suspects Gardner might be behind the day's events. Jack, with help from Wayne secretly sets up a meet with Henderson to exchange Evelyn's evidence for her daughter, during which Jack manages to save Amy and kill Henderson's henchmen but not stop Henderson himself from escaping. Audrey goes along with a plan to remove Buchanan from power in order to keep Chloe at CTU to assist Jack. Jack then learns from Evelyn that it is not Vice President Gardner who is responsible for the day's events, but President Logan.

New discoveries regarding the day's horrific events keep Jack away from CTU as he learns from Evelyn that the evidence against Logan is stored at a bank. He and Wayne kidnap the manager with the help of Chloe as they attempt to retrieve it but Henderson, having tracked down Evelyn Martin, discovers Jack's location and sends a team to kill him and retrieve the evidence. Jack and Wayne manage to escape, setting off a bank alarm which brings LAPD to the area causing a firefight between Henderson's men and LAPD. Meanwhile CTU is absorbed by Homeland Security. Chloe and Audrey team together to assist with Jack's field operations and the investigation takes an unexpected turn.

Although Logan claims the country is no longer under attack, martial law is still enforced, and CTU remains under intense pressure to find the remaining fugitives. Meanwhile, Jack, Audrey, and Secretary of Defense James Heller (William Devane) reunite for a pivotal meeting. Heller has Jack detained, believing that if Logan's complicity was made public and prosecuted, it would destroy the integrity of the U.S. presidency. Heller, attempts to force Logan to step down as President, but Henderson locates Jack and manages to recover the recording, which forces Heller to stand down.

Jack recaptures Henderson, but his men have been shadowing Heller with a helicopter and sniper and threaten to kill him unless released. Heller, chooses to drive his car off the cliff into a lake rather than allow Jack to release Henderson, who is taken into custody. Tensions at the President's retreat escalate as Martha grows suspicious of her husband. Meanwhile Curtis Manning helps in the struggle against Henderson. A surprising new ally in the form of Karen Hayes joins Buchanan and O'Brian as they continue to assist Jack, who boards a diplomatic flight searching for an associate of Henderson's, to whom the evidence has been passed on.

Although Karen wishes to help Buchanan, he is still taken into CTU to maintain the cover while Chloe escapes. Meanwhile, Jack hijacks a plane in search for the recording that Henderson handed off and discovers that the Henderson's associate is actually the co-pilot, and not the passenger he apprehends. At the retreat, Martha grows increasingly unstable, causing further dispute between Logan and Mike, who begins to suspect something is wrong. Graem (Paul McCrane) continues to manipulate Logan into taking drastic measures, and suggests that the flight Jack has boarded, be shot down to prevent him recovering the evidence.

With Graem's help, Logan decides to launch one final attempt to destroy the plane Jack Bauer has hijacked, thus securing the recording that implicates him in the conspiracy surrounding the day's events. The military scrambles an F-18 from Point Mugu with orders to shoot down the plane. At CTU, several agents begin working with Jack, Chloe, and Buchanan. Jack is able to land the plane, having recovered the recording and is extracted by Curtis as Logan sends in the military to take him into custody. Tensions continue to rise in Logan's retreat as the President, on discovering that Jack has escaped with the recording, contemplates suicide. Meanwhile, Bierko is transported to another facility, and Miles Papazian (Stephen Spinella), one of Karen Hayes' people, attempts to further his career by erasing the recording to gain a favor from Logan.

Jack, Chloe, Buchanan, and Karen Hayes (Jayne Atkinson) realize that the recording was erased by Papazian. However, CTU is soon forced to switch tracks and focus on a new threat: the team transporting Bierko is attacked and Bierko escapes, and he plans to seize another powerful weapon to use against the US — an armed Russian submarine being serviced in the Port of Los Angeles as part of the US-Russian treaty. Using the last canister of Sentox nerve gas to kill the submarine crew, Bierko and his men take over and prepare to launch the submarine's missiles. With the new threat quickly approaching, Jack must use Henderson in a delicate sting operation. Meanwhile, the drama at the retreat turns deadly as Martha discovers Aaron Pierce about to be killed, and rescues him, killing Logan's henchman in the process.

Jack and Henderson, with help from a surviving member of the submarine crew, infiltrate the submarine and secure a missile submarine seized by Bierko, killing Bierko in the process. Afterward Henderson attempts to escape and pulls a gun on Jack, believing that Jack would not have honored his original deal to help Henderson 'disappear' in exchange for his help. Henderson pulls the trigger but the gun is empty, and Jack executes Henderson, exacting his revenge for David, Tony and Michelle. Martha and Pierce form an alliance with Mike and eventually also bring in Jack to help. Meanwhile, Logan prepares to leave for Palmer's sendoff, which Jack exploits to his advantage to acquire evidence against him.

Mike, Aaron, Martha, and Jack conspire to kidnap Logan as part of a plan to secure a confession about the original conspiracy behind the day's events by posing as an HMX-1 pilot, and taking control of the helicopter. Jack takes him to an abandoned building and attempts to acquire a confession with help from Chloe and Morris — Chloe's ex-husband. When Jack's gambit fails, Martha and Chloe initiate a back-up plan to incriminate the President — Jack had planted a mic on Logan, who during an argument with Martha, implicates himself in the day's events. This is broadcast to the Attorney General and Logan is duly arrested by Secret Service. Meanwhile, Jack's plan to disappear again is ruined when the Chinese abduct him for the events that occurred 18 months prior at the Chinese consulate and put him onboard a ship to China.